Why Are My Asparagus Leaves Curling?

Asparagus beetle

Asparagus beetle (Crioceris asparagi) is the most significant pest of UK asparagus beds. The adult is a striking small beetle with red, yellow, and black markings. Both adults and larvae feed on asparagus: adults attack the spears during harvest season, gnawing the skin and causing the tips to droop and discolour, and then move on to strip the green tissue from the fronds through summer. Defoliated fronds turn brown and curl as the tissue dies. Severe defoliation in July and August significantly reduces the size of the crown and the following season's spear production. The beetles overwinter in plant debris and soil near the asparagus bed and emerge in April.

What to do

• Inspect asparagus fronds daily from late April for adult beetles and the small, dark, slug-like larvae. Both adults and larvae can be hand-picked into a container of soapy water and disposed of. This is the most practical and effective control on a small garden scale.
• Remove and dispose of all asparagus fern debris in autumn after cutting down, rather than leaving it near the bed. This removes overwintering sites for the beetles and significantly reduces the population the following spring.
• Encourage natural predators: birds, particularly robins and blackbirds, eat asparagus beetle larvae effectively if you do not disturb them when they are foraging around the bed.
• Pyrethrin-based insecticide spray can be used on badly infested beds as a last resort: apply in the evening to avoid harming bees.

Asparagus rust

Asparagus rust (Puccinia asparagi) is a fungal disease that produces small, orange-brown, powdery pustules on the stems and fronds of asparagus from midsummer onward. The tissue around the pustules turns yellow and the fronds curl as the infection spreads. Heavy infections cause premature yellowing and dieback of the fronds from July onward, reducing the plant's ability to build energy reserves before autumn. Rust is most common in warm, humid summers and in sheltered beds with poor airflow.

What to do

• Choose rust-resistant asparagus varieties where possible: modern male varieties such as 'Gijnlim', 'Backlim', and 'Jersey Knight' have significantly better rust resistance than older varieties.
• Remove and destroy badly infected fern material promptly; do not compost it. Remove all fern debris at the end of the season to reduce overwintering spore populations.
• Ensure the bed has good airflow: avoid planting asparagus in sheltered, humid positions. Remove weeds and competing vegetation that would reduce airflow through the bed.

Drought

Asparagus is relatively drought-tolerant once established, with deep, extensive root systems that access moisture from depth. However, drought during the period when the fronds are actively growing in summer reduces frond development and therefore the energy reserves stored in the crown. Young crowns planted in the last 2 to 3 years are more sensitive to drought than established beds.

What to do

• Water asparagus beds during extended dry spells in summer, prioritising young beds in their first 3 years and established beds during drought in July and August when the fronds are at their most productive.
• Apply a mulch of well-rotted compost or manure in spring, which conserves soil moisture and feeds the crowns simultaneously.

Violet root rot

Violet root rot (Helicobasidium brebissonii) occasionally affects asparagus beds, producing a purple-violet fungal growth on the roots and crown. Infected plants produce weak, yellowing fronds that may curl and collapse. The disease spreads slowly through the bed via root contact and contaminated soil. It is favoured by acidic soils and is more common on beds that have been established for many years.

What to do

• Lime the asparagus bed if the soil is acidic: aim for a pH of 6.5 to 7.5. Violet root rot is significantly less severe in neutral to slightly alkaline soil conditions.
• Dig out and destroy badly affected crowns. Do not replant asparagus on affected ground for several years.

Wind damage

Asparagus fronds grow tall (up to 120 to 150 centimetres) and are vulnerable to wind damage in exposed gardens. Strong winds snap the main stems or abrade the fronds, causing them to brown and curl along the damaged areas. Wind-damaged fronds photosynthesize less effectively, reducing the energy stored in the crown.

What to do

• Support asparagus fronds in exposed gardens with canes and twine along the length of the bed: run two or three rows of horizontal string or twine along the sides of the bed at different heights to support the fronds as they grow.
• If the site is very exposed, consider planting a low windbreak hedge on the prevailing wind side of the bed, set far enough back not to shade the asparagus.

Frequently asked questions

Why are my asparagus leaves curling and yellowing?

Asparagus foliage (the feathery fern-like fronds that develop after the spears are harvested) curls and yellows most often from asparagus beetle damage or asparagus rust. Asparagus beetle (Crioceris asparagi) adults and larvae strip the green tissue from the asparagus fronds, causing them to brown and curl as the tissue dies. The beetle also attacks the spears during harvest season. Asparagus rust (Puccinia asparagi) produces orange-brown pustules on the fronds that yellow and curl the surrounding tissue. Both problems affect the ability of the plant to photosynthesize and build energy reserves in the crown for the following season's spears.

How long before I can harvest asparagus?

Asparagus requires patience: do not harvest any spears in the first year after planting crowns, and harvest only lightly (2 to 3 spears per plant) in the second year. The first full harvest is in year 3, when the bed is properly established. From year 3 onward, harvest all spears that emerge between April and mid-June, then stop cutting and allow the fronds to develop for the rest of the season. Asparagus crowns planted as one-year-old crowns (the most common starting point in the UK) reach full production by year 3 to 4 and continue to crop well for 20 to 25 years if the bed is properly maintained.

Should I cut asparagus ferns down in autumn?

Do not cut asparagus ferns down while they are still green: the fronds continue to photosynthesize and build energy reserves in the crown through autumn, which directly determines the quality and quantity of the following season's spears. Only cut the fronds down once they have turned fully yellow or brown in late autumn or early winter (typically November). Cut the stems to within a few centimetres of the soil surface and remove all the cut material from the bed. If asparagus beetle has been a problem, remove the old fern completely rather than leaving it on site, as the beetles overwinter in the crop debris.

Why are my asparagus spears thin?

Thin asparagus spears are most often the result of a young or under-established crown, overcrowding, or harvesting too heavily in previous seasons. The thickness of asparagus spears is directly related to the size of the crown that produces them: small crowns produce thin spears, large crowns produce thick spears. An asparagus bed that has been harvested too hard in previous years without allowing sufficient fern development will gradually weaken, producing thinner and fewer spears each season. Allow the ferns to grow unchecked for the full season after the harvest period ends each June to rebuild the crown. On an established bed, thin spears indicate the crowns need feeding: apply a general balanced fertiliser in spring and a mulch of compost or well-rotted manure each autumn.